1. Field of the Invention
The present invention generally relates to devices and methods for measuring fluid properties for oilfield applications and other industries, e.g., chemical and food industries. In particular, the invention relates to the measurement of the density of microfluidic volumes of fluids for surface and downhole oilfield applications.
2. Background of the Invention
Understanding fluid density and other fluid properties downhole is paramount to petroleum exploration as it enables one to differentiate between oil, gas and water [W. D. McCain, Jr., The Properties of Petroleum Fluids, 2nd ed. (1990)]. Furthermore, it allows one to locate the oil-water contact line and hence the thickness of the pay zone of a formation. As a consequence, it is a must that robust sensors be developed that can accurately measure fluid density and other fluid properties in a harsh environment found in an oilwell. Oilfield pressures downhole typically range as high as 15,000 psi with temperatures as high as 150° C., though wells exist with far more extreme conditions, especially offshore. A further challenge in downhole fluid analysis is that it is a challenge to obtain large quantities of representative downhole fluids due to the ever-present contamination, whether drilling mud or formation water [O. C. Mullins, M. Hashem, H. Elshahawi, G. Fujisawa, C. Dong, S. Betancourt, T. Terabayashi, Petrophysics 46, 302 (2005)]. Hence sensors that can operate with small quantities of fluid provide a great advantage. Further, Schlumberger has made some progress on miniaturizing vibrating tube densitometers as noted in J. G. Blencoe, S. E. Drummond, J. C. Seitz, and B. E. Nesbitt, International Journal of Thermophysics, 17, 179 (1996).
The vibrating tube densitometer has a well-deserved reputation as the world's most accurate technology for measuring fluid density among other things, both at ambient conditions and at elevated temperature and pressure [J. G. Blencoe, S. E. Drummond, J. C. Seitz, and B. E. Nesbitt, International Journal of Thermophysics, 17, 179 (1996) and R. Laznickova and H. Huemer, Meas. Sci. Technol. 9, 719-733 (1998)]. It is noted that the accuracy is in part due to the simplicity and the robustness of the underlying physics as well as its suitability to a wide range of temperature and pressure. For example, a measurement is performed by filling the tube with the fluid to be measured and the tube is excited at its resonant frequency by a piezoelectric or electromagnet actuator. Motion and hence the resonant frequency is measured with a piezoelectric transducer or an electric pickup coil. Adding the mass of such transducers decreases the sensor's sensitivity to fluid density as well as adding to the complexity of the device. Furthermore, the temperature dependence of these transducers must be incorporated into the interpretation.
Density of a single phase fluid can be one of the fundamental physical parameters required to describe fluid flow, either within the reservoir or borehole, as well as determine both the properties of the surface facilities and the economic value of the fluid as noted above; it is also required to provide the volume translation factor for cubic equations of state that are then used for reservoir simulator. A measure of the single phase fluid density within a sampling tool provides a real-time in situ determination of bore-hole fluid contamination as well as economic value. Immiscible fluids are required or a separator may be needed to provide the single phase fluid. Measurements with emulsions may be performed and it then becomes a matter of knowing the volume of each co-mingled phase before the density of the oil can be extracted; this can be achieved with, as an example, coincidence gamma-ray attenuation measurements with a micro Curie source as described by Schlumberger. For most applications outside of equation of stak analysis, an expanded uncertainty in density of ±0.01·ρ can be sufficient.
Moreover, there are many methods that can be used to measure fluid density in a laboratory, for example some of these methods are described by the following: 1) Wagner et al. [J. W. Density in Experimental Thermodynamics Vol. VI, Measurement of the Thermodynamic Properties of Single Phases , Ch. 5 , Goodwin, A. R. H., Marsh, K. N., Wakeham W. A. Eds.; Elsevier for International Union of Pure and Applied Chemistry: Amsterdam, 2003; pp 127-235]; 2) Wagner and Kleinrahm [Densimeters for very accurate density measurements of fluids over large ranges of temperature, pressure, and density. Metrologia 2004, 41 , S24-S39]; and 3) Kuramoto et al. [Accurate density measurements of reference liquids by a magnetic suspension balance. Metrologia 2004, 41 , S84-S94]. However, Fujii further describes absolute density standards [Absolute Density Standards in Experimental Thermodynamics Vol. VI, Measurement of the Thermodynamic Properties of Single Phases, Ch. 5, Goodwin, A. R. H., Marsh, K. N., Wakeham W. A. Eds.; Elsevier for International Union of Pure and Applied Chemistry: Amsterdam, 2003; pp 191 to 208 , and Present state of the solid and liquid density standards. Metrologia 2004, 41, S1-S 15].
Of the above-mentioned methods, the methods that appear most appropriate for down-hole applications are those that do not rely on the knowledge of the orientation of the transducer with respect to the local gravitational field. These methods are based on determining the resonance frequency of a vibrating object and have been summarized by Majer and Pádua [Measurement of Density with Vibrating Bodies in Experimental Thermodynamics Vol. VI, Measurement of the Thermodynamic Properties of Single Phases, Ch. 5 , Goodwin, A. R. H., Marsh, K. N., Wakeham W. A., Eds.; Elsevier for International Union of Pure and Applied Chemistry: Amsterdam, 2003; pp 158-168] and in particular Stansfeld with descriptions of devices for use at well-heads and pipelines [In situ Density Measurement in Experimental Thermodynamics Vol. VI, Measurement of the Thermodynamic Properties of Single Phases, Ch. 5 , Goodwin, A. R. H., Marsh, K. N., Wakeham W. A., Eds.; Elsevier for International Union of Pure and Applied Chemistry: Amsterdam, 2003; pp 208-225].
There are many geometrical arrangements that have been reported for oscillating object densimeters with the fluid contacting either the outer or inner surface of, what is usually a metallic object. When the fluid is in contact with the outer surface, the measurement is usually considered intrusive when operated at elevated pressure, but when the fluid is inside the tube it is a non-invasive measurement. Once the particular device has been selected it remains a task to develop a working equation, based on the principles of physics, that relates the measured quantity (in this case frequency) to density and provide a measurement with an expanded (k=2 or 95% confidence interval).
In view of tubulars used within a Modular Dynamics Tester (MDT) a measure of density may be best obtained by a vibrating tube. The vibrating U-tube is one of the plausible geometries, however there are others [In situ Density Measurement in Experimental Thermodynamics Vol. VI, Measurement of the Thermodynamic Properties of Single Phases, Ch. 5 , Goodwin, A. R. H., Marsh, K. N., Wakeham W. A., Eds.; Elsevier for International Union of Pure and Applied Chemistry: Amsterdam, 2003; pp 208-225]. Tubes offer another advantage for wire-line (as well as other tool conveyance methods and MWD) in that they can be of low mass and be well suited to sustaining mechanical shock; rapid changes in local acceleration and the resultant application of large forces. Indeed, as the internal diameter of the tube decreases so does the outer diameter while still maintaining the ability to sustain a pressure difference across the tube from within. The type of material used to construct the tube and the elastic properties will determine the absolute value of the pressure difference sustainable by a tube wall.
Most densitometers are calibrated using a calibration fluid having a known density wherein the density is measured at a specified temperature. The problem with trying to obtain a density measurement outside of a laboratory/controlled environment is that the density of most fluids varies with temperature. Presently, many currently designed densitometers require that the temperature of the calibration fluid must be controlled prior to the fluid being injected into the densitometer for calibration. This means that the calibration fluid must be in a container that is temperature controlled so that the fluid will be held at a constant temperature. It is noted that the piping of the fluid from the container to the densitometer must also be temperature controlled to ensure that the temperature being pumped does not change in temperature during the transition. Thus, controlling the temperature of stored calibration fluid along with ensuring the temperature of the fluid does not change while the fluid is being pumping to the measuring device, can be both expensive and a difficult process.
There are known examples of varying types of densitometers or the like. For example, U.S. Pat. No. 4,170,128 issued to Kratky et al. (hereafter “KRATKY”), incorporated by reference herein in its entirety, shows a device comprising a U-shaped bending type oscillator connected with a tensioned body responsive to temperature and pressure. However, the above reference has many drawbacks, such as a geometry that requires a large quantity of fluid in order to completely replace a fluid that is initially present in the tube with a second one. Furthermore, it has an internal volume closer to milliliters rather than microliters, and a geometry that it is not optimized to operate at large pressures.
U.S. Pat. No. 7,263,882 issued to Sparks et al. (hereafter “SPARKS”), incorporated by reference herein in its entirety, shows a densitometer relating to chemical concentrations, including those of fuel cell solutions that can be measured by sensing changes in fluid density as a fluid sample flows through a microchannel within a resonating tube of a Coriolis-based microfluidic device. While the SPARKS device discloses the use of Coriolis-based microfluidic devices for sensing the mass flow rates and densities of gases and gas mixtures, many more improvements in the sensitivities of such devices are necessary to fully realize the capabilities of such devices. Further, the SPARKS device discloses an oscillating tube densitometer that is fabricated out of silicon and operates with microliter volumes of sample fluid. The SPARKS device is unable to operate at pressures much above ambient pressures as the vibrating element consists of a thin-walled silicon tube. Moreover, the SPARKs device is not a device operable downhole and is limited to low pressure and low temperature.
U.S. Pat. No. 6,378,364 issued to Pelletier et al. (hereafter “PELLETIER '364”), incorporated by reference herein in its entirety, shows a densitometer for determining fluid properties from vibration frequencies of a sample cavity and a reference cavity. The measurement device of PELLETIER '364 includes a sample flow tube, a reference flow tube, vibration sources and detectors mounted on the tubes, and a measurement module. The sample flow tube receives a flow of sample fluid for characterization. The reference flow tube is filled with a reference fluid having well-characterized properties. The reference flow tube may be pressure balanced to the same pressure as the sample. The measurement module employs the vibration sources to generate vibrations in both tubes. The measurement module combines the signals from the vibration detectors on the tubes to determine properties of the sample fluid, such as density. In particular, to determine the sample fluid density, the measurement module of PELLETIER '364 measures the difference between resonance frequencies of the sample flow tube and the reference flow tube. The density can then be calculated according to a formula. However, the main drawback for the Pelletier device, among other things, is that it requires milliliter-sized volumes as fluid for operation, predominantly discusses measurements with respect to a second tube referred to as a standard, along with being disclosed as a large device. Further, another major drawback of the PELLETIER '364 reference is that it is impractical as well as not commercially viable due to the use of the reference frequency originating from the idea that there will be a second vibrating tube in the tool, filled with a fluid of known properties or a vacuum. Further still, the PELLETIER '364 reference requires the reference frequency due to the structure of the device, e.g., affixing a magnet to the tube and detecting with a pickup coil.
U.S. Pat. No. 6,543,281 B2 issued PELLETIER (hereafter “PELLETIER '281), incorporated by reference herein in its entirety, shows a downhole vibrating tube densitometer. However, the downhole vibrating tube densitometer of PELLETIER '281 has an inner diameter of a tube on the order of 5 mm, leading to a sensor volume of tens of milliliters at a minimum. The PELLETIER '281 has other drawbacks, as mentioned above, utilizes milliliter scale volumes of fluid for operation and requires either the excitation or detection components to be clamped to the tube, thereby decreasing the sensor's sensitivity.
U.S. Published Patent Application US 2008/0156093 to Permuy et al. (hereafter “PERMUY”), is commonly assigned to the same assignee of the present application and incorporated by reference herein in its entirety, and shows a commercialized densitometer (InSitu Density) for flowline applications. The PERMUY devices shows a sensor device based on the use of mechanical elements put into vibration in the fluid to be measured. However, the sensor device of PERMUY requires several milliliters of fluid at a minimum.
Anton Paar is often recognized as the world leader in laboratory vibrating tube densitometers. A recently introduced model is now able to operate at 20,000 psi and at elevated temperatures. However, this device requires milliliters of fluid to measure density. The Anton Paar reference discloses a device that does not incorporate a pressure housing so as to operating in a pressure environment. Further, the Anton Paar reference requires milliliter scale volumes of fluid for operation which is not suitable for below ground environments.
More recently it has been shown that actuation can be achieved by placing part or all of the vibrating tube into a magnetic field and by passing oscillatory current through the tube body itself [J. Herrero-A´ lvarez, G. Gonza´lez-Gaitano, and G. Tardajos, Rev. Sci. Instrum. 68, 3835 (1997) and R. F. Chang and M. R. Moldover, Rev. Sci. Instrum. 67, 251 (1996)]. For example, the Chang and Moldover reference (hereafter “CHANG”) discloses a vibrating tube design that eliminates electromagnets and appendages attached to tube of the densimeter, and can operate at elevated temperature. CHANG discloses measurements of density of toluene between 298 K (about 900 kg×m-3) and 575 K (about 600 kg×m-3) at pressures below 13.8 MPa. However, there are many drawback to this device since, first there is no disclosed pressure housing, and secondly, the disclosed device would suffer from electrical issues since no electrical isolators or similar like devices have been incorporated.
Therefore, there is a need for methods and devises that overcome the above noted limitations of the prior art. By non-limiting example, devices and methods that can provide a high-accuracy densitometer which is capable of operation under the high temperature, pressure, shock and vibration conditions encountered in a wellbore; which uses a fluid sample volume equal to or less than 100 microliters; and which effectively eliminates the errors associated with the effects of temperature and pressure on the system as well as suppress electrical noise coming from exterior influences positioned exterior to the device.